Not applicable.
Not applicable.
(1) Field of the Invention
This invention relates generally to antennas, and more specifically, to antennas comprised of arrayed elements having nominally omnidirectional patterns in two dimensions.
(2) Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 37 CFR 1.98.
Array antennas are employed in diverse application areas, including direction finding, null forming, space division multiple access, multipath canceling, and mapping. The elements of the array are chosen based upon element gain and coverage, either two-dimensional or three-dimensional.
An array antenna may be comprised of a plurality of monopole antennas or monopole-like elements. A monopole antenna is an antenna consisting of a straight conducting rod, wire, or other structure oriented nominally perpendicular to a ground plane and fed near the junction of the structure and the ground plane. The class of monopole antennas in general also includes short dipoles and vertically polarized patch antennas. As is well known in the art, the radiation pattern of the monopole antenna is two dimensional, transverse to the longitudinal axis of the monopole, with the peak normal to the axis of the monopole. The coverage area of the antenna is approximately uniform in the horizontal plane. By utilizing an array of monopole antennas, a uniform controlled coverage is obtainable, having a steerable pattern function, and gain that is dependent upon the number of elements and control implementation. Antennas comprised of arrayed elements having nominally omnidirectional patterns in two dimensions are designed to optimize one or more of the following characteristics: peak gain, high front-to-back ratio, low omnidirectional ripple in gain, uniform sidelobe and backlobe level, and high packing ratio with circular symmetry. These characteristics may be optimized to varying degree by using classical array topologies of the prior art. These array topologies include square lattices, hexagonal lattices, and random arrays, among others. However, square and hexagonal lattices are constrained to a fixed number of antenna elements. Random arrays are difficult to manufacture since the element locations are not defined by deterministic functions. Thus, the need exists for an array antenna that exhibits high gain, high front-to-back ratio, low gain ripple, good sidelobe and backlobe characteristics, and high packing ratio while at the same time being easily manufactured having an arbitrary number of elements.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of instrumentalities and combinations particularly pointed out in the appended claims.
In view of the aforementioned needs, the invention contemplates an Archimedes spiral array antenna comprised of an arbitrary number of arrayed elements having nominally omnidirectional patterns in two dimensions spaced at a fixed fraction of wavelengths apart on the spiral, with the first element located at or near the center of the spiral and adjacent arcs of the spiral located at a fixed fraction of wavelengths apart.
The topology of this configuration is unique from three standpoints. First, the element-to-element spacing, location of the first element, and arc-to-arc distances are arbitrary. Second, the array exhibits aperiodic differential phase characteristics across its aperture in two dimensions, independent of the number of elements. Third, the locations of the array elements are easy to define.
The arc-to-arc spacing distances, location of the first element, and element-to-element spacing are arbitrary, providing three degrees of freedom for the control of grating lobe and backlobe characteristics. The defining equation of the spiral, R=kA, employs k to control the spacing between adjacent arcs of the spiral, where R is the radial measure and A is the angular measure in a circular coordinate system. The first element is located at initial position (R0,A0). The element-to-element spacing is d=|(Rn,An)xe2x88x92(Rnxe2x88x921,Anxe2x88x921)|, the Euclidean distance between adjacent elements, for all elements n={0, 1, . . . , mxe2x88x921}, and m total elements. These three parameters control array area, packing ratio, sidelobe levels, and gain ripple. Both k and d are generally chosen so that elements and spiral arms are on the order of one-half wavelength apart; the first element is generally at or near the origin of the spiral.
The Archimedes spiral array can be designed to exhibit aperiodic differential phase characteristics across its aperture in two dimensions, independent of the number of elements. This characteristic forms the basis of its excellent backlobe performance, in particular for a relatively small number of elements, for example eight or more. Because the number of elements is arbitrary, the Archimedes spiral array antenna is ideally suited to applications where the system architecture requires a specified number of elements. Conversely, conventional arrays defined by periodic grids require a specific number of elements to fill the periodic grid and are hence constrained to those numbers.
The locations of the elements for an Archimedes spiral antenna are easy to define, contributing to ease of manufacturing. The simplicity of defining locations is due to the fact that the defining equation is a deterministic function. The antenna array of the present invention displays the same desirable performance characteristics as a random array but has a higher packing density and is easy to manufacture.
The antenna array of the present invention is easily distinguished from prior art spiral antennas. The typical prior art spiral antenna is a logarithmic spiral. The prior art spiral antenna is usually etched or attached to a substrate. The prior art spiral antenna designs do not have monopole elements, and consequently are not arrays, and the field is nominally perpendicular to the plane of the spiral.
In contrast, the present invention is directed to an array antenna with a plurality of monopole or similar elements arranged along an Archimedes spiral topology. The monopole elements are extending away from the plane of the spiral. The electric fields are parallel to the monopole elements (not the plane of the spiral), the peak gain patterns of which are normal to the axis of the spiral elements.
While the present invention has been described in a preferred embodiment utilizing monopole elements, those skilled in the art should readily appreciate that other arrayed elements having nominally omnidirectional patterns in two dimensions such as dipoles or patch antennas are also suitable.
Among those benefits and improvements that have been disclosed, other objects and advantages of this invention will become apparent from the following description taken in conjunction with the accompanying drawings. The drawings constitute a part of this is specification and include exemplary embodiments of the present invention and illustrate various objects and features thereof.